Gregorian calendar
The Gregorian calendar, which is currently used in most countries of the world, was derived from the Julian calendar. In the Julian calendar, a mean length of 365.25 days for the year is adopted. The calendar year is adjusted to this mean value by inserting an intercalary day every four years. The intercalary or leap year has 366 days and each of the other three years has 365 days. The year with a numerical designation divisible by 4 would be a leap year. The mean Julian calendar year exceeds the length of the Earth's rotation around the Sun (365.2422 days) by about 11 m 14s.
In 1582, Pope Gregory XIII revised the Julian calendar and adopted a different rule for leap year by omitting the intercalary day in century years that are not divisible by 400. In the Gregorian calendar, there are 97 leap years (366 days a year) and 303 ordinary years (365 days a year) for every 400 years so that the mean Gregorian calendar year is 365.2425 days, quite close to the length of the Earth's rotation around the Sun of 365.2422 days.
Is the Year 2000 a leap year?
According to the leap year rules of the Gregorian calendar, 2000 is divisible by both 4 and 400 and thus the Year 2000 is a leap year. However, the century years 1700, 1800 and 1900 are not leap years as the figures are not divisible by 400.
